While the present invention can be used in any paper feeding marking system, it will be described herein for clarity as used in electrostatic marking systems such as electrophotography or xerography.
By way of background, in marking systems such as xerography or other electrostatographic processes, a uniform electrostatic charge is placed upon a photoreceptor belt or drum surface. The charged surface is then exposed to a light image of an original to selectively dissipate the charge to form a latent electrostatic image of the original. The latent image is developed by depositing finely divided and charged particles of toner upon the belt or drum photoreceptor surface. The toner may be in dry powder form or suspended in a liquid carrier. The charged toner, being electrostatically attached to the latent electrostatic image areas, creates a visible replica of the original. The developed image is then usually transferred from the photoreceptor surface to an intermediate transfer belt or to a final support material such as a paper sheet. When the paper is fed to the system from a paper stack of a feeder mechanism, some papers could be off the home position by many mm and these fed paper sheets need to be rapidly and properly fed.
In high speed commercial printing machines, precise timing and positioning of fed paper is critical to producing consistently high quality images. Generally, in the sheet feeding assembly, an apparatus blows gas or air against a sheet stack on a sheet stacking unit; then the sheet feeding apparatus suctions an uppermost one of the sheets to a conveyance belt via take-away rollers (TAR) to be carried through the marking system for contact with the latent image on the photoreceptor. The sheet feeding apparatus is precisely controlled by appropriate sensors and controllers to ensure that the acquisition time (to rollers and conveyance belt) is exactly uniform and according to the desired timing.
Sheet feeding assemblies used in marking systems are disclosed in U.S. Pat. No. 7,461,839 (Ikeda), and U.S. Pat. No. 7,258,336, the disclosures of these two patents are incorporated by reference into the present disclosure. In U.S. Pat. No. 7,461,839 a sheet-feeding assembly similar to the present invention is disclosed using a suction assembly where the suction openings are closed after contact with the uppermost sheet of the sheet stack; the conveyance belts are driven after the suction-detecting hole is closed according to the negative pressure in the suction duct. A primary distinction from the present invention is that in Ikeda's system the closing of the suction openings by the upper most sheet also causes the cap on the suction-detecting hole to close, while in the present invention the pressure difference between the interior and exterior of the feedhead due to the suction openings being closed creates a force that lifts the cap away and opens the suction-detecting hole. The advantage in the present invention is that by using the pressure difference between the feedhead interior and ambient to open the suction-detecting hole instead of relying on a pressure difference due to a buildup of air flow between the cap and hole to close the suction-detecting hole, it takes much less time than closing the suction-detecting hole which could present a problem in a high speed printing apparatus. Thus, it takes substantially less time to open the suction-detecting hole which is a huge advantage in high speed systems. Also, a further advantage is that the design of the current invention is more straight forward as there is no need to carefully control the distance between the open cap and the suction-detecting hole such, which would be necessary with the prior invention. The feeder of the present invention can equally be used in monochrome or color high speed systems.
In addition, currently, vacuum corrugated feeders operate via open loop control in which actuations or closing of ports of the various mechanisms occur according to a preset timing chart. More specifically, it is expected that the acquisition of a single sheet by the feedhead will occur within a preset period of time after which the feedhead transports the sheet to the take away rolls. If the sheet was not acquired in time, a misfeed will be declared. While it would be advantageous to detect the acquisition of the sheet by the feedhead, the pressure sensor needed to perform this function is relatively expensive.